1. Field of the Invention
This invention relates to the production of curable silicone rubber. More particularly, this invention relates to a method for producing liquid silicone rubber bases containing relatively large amounts of filler without the need for a plasticizer.
2. Description of the Prior Art
Silicone rubber moldings are conventionally produced by kneading a silicone rubber composition, which principally consists of a polyorganosiloxane gum and an inorganic filler such as reinforcing silica, together with an organic peroxide followed by crosslinking and curing during molding. This method requires a complicated process of softening or blending using a rubber mill prior to the molding step.
A method for molding liquid silicone rubber that avoids the foregoing complicated process has recently become widespread. The molding is produced by curing a liquid polyorganosiloxane having silicon-bonded vinyl groups and a relatively low degree of polymerization using a polyorganohydrogensiloxane in the presence of a platinum-containing catalyst. This method offers the advantages of simplifying the entire process because the composition is flowable before curing. The process is so simple that it can be applied to injection molding and can be automated.
However, liquid silicone rubber compositions suitable for molding must be flowable even though they contain large amounts of reinforcing fillers. For this reason, a plasticizer must be added such as hexamethyldisilazane, diphenylsilanediol, a cyclic polydimethylsiloxane or a polydimethylsiloxane exhibiting silanol or alkoxy terminal groups and a degree of polymerization of 20 or less. As a result, the complexity of the composition or process is increased and production costs become high.
An objective of the present invention is to eliminate the above-mentioned disadvantages by providing a method for producing a flowable liquid silicone rubber base using a simple process that does not require the addition of any plasticizer.